Analysis of the composition of a sample (e.g., the element and/or chemical species concentration in a thin film formed on a substrate) is necessary in the manufacture of many different types of devices (e.g., electronic and optical electronic devices). For example, it is necessary to determine the composition of gate oxide films formed in known semiconductor integrated circuit devices, such as processing devices and memory devices. Increases in the density of such devices on an integrated circuit chip and reduction in device dimensions require the advancement of production and characterization technologies of the materials used to fabricate such devices.
For example, recent developments in the fabrication of semiconductor devices may employ shallow implant and/or other ultra-thin structures. In one particular example, gate oxide layers are becoming very thin films, typically less than about 10 nanometers in thickness. Such thin films are difficult to characterize. Such structures will require characterization techniques that have improved sensitivity over conventional characterization techniques. Further, such techniques may also require the characterization to be performed with ample speed.
Various techniques have been used for surface analysis of trace and/or major components in such materials. For example, several of such methods include secondary ion mass spectrometry (SIMS), x-ray photoelectron spectrometry (XPS) (also known as electron spectroscopy for chemical analysis (ESCA)), and Auger electron spectrometry (AES). Such techniques are sensitive to the near-surface region of a material. However, these techniques do permit measurement of material properties as a function of depth beneath the surface through depth profiling.
In typical depth profiling, for example, continuous or periodic ion beam sputtering removes material from the surface of a sample to expose progressively deeper material at one or more various depths of the sample for further measurement and/or analysis. Generally known sputter rates may be used to determine the depth at which the surface measurements are completed. As such, a characterization of the sample as a function of depth beneath the surface can be attained.
However, such techniques and/or the parameters at which such techniques are carried out are inadequate in many respects with regard to characterization of various types of samples and, in particular, for example, with respect to characterization of thin films, e.g., thin gate oxide films. For example, SIMS, which has a very small sampling depth, is used to look for low level dopants and impurities in thin films (e.g., thin films less than 10 nanometers) because of this technique's extreme surface sensitivity. However, SIMS is problematic in that it is difficult to quantify major constituents of a thin film because of matrix effects that impact the secondary ion yield of different chemical species.
Further, AES has also been used for thin film characterization. However, the high intensity electron beam used to make Auger measurements can alter the apparent composition of a thin film by causing chemical damage or the migration of elements within the thin film. For example, nitrogen is known to migrate to the interface of an oxynitride (ONO) film provided on silicon.
XPS, or ESCA, depth profiling has been used for thin film characterization. However, when used, the analyzer of the system is typically positioned at a low analyzer angle relative to the sample surface such that depth resolution is enhanced. Such a low analyzer angle is typically less than 20 degrees. Use of a low analyzer angle generally results in a slow characterization process and also may result in problems associated with placement of the analyzer of the characterization system relative to the sample being analyzed.
Further, ESCA surface measurements taken at various surface to analyzer angles have been used to examine thin films. Such examination has been performed with the use of a mathematical method to obtain depth distributions. However, the mathematical method has no unique solution.
Further, optically based ellipsometry methods have also been used to monitor thin film thickness and composition. However, these methods cannot measure elemental or chemical distributions within the thin film and cannot provide a dose measurement of minor added constituents. In addition, such methods are not capable of providing reliable results for thin films less than 2 nanometers in thickness.
Also, transmission electron microscopy (TEM) combined with electronic energy loss spectroscopy (EELS) measurements can provide thickness and composition distribution information. However, these methods are not practical for process monitoring because of the cost and time needed to prepare samples to be analyzed thereby.
Many of such techniques described above for characterizing thin films are invasive techniques, e.g., they involve destruction of at least one or more portions of the sample. Such techniques, e.g., those that use removal of material during depth profiling, are sufficient in many circumstances, e.g., research and development, product testing, etc., but do not provide for the ability to quickly analyze a thin film such as is necessary in production processes. For example, in such production processes, a thin film being formed typically needs to be analyzed so that such information can be used for production control, product test, etc., without loss of product due to invasive characterization of such films.
For example, nitrogen doped or nitrided silicon oxide is one material that is used as a gate oxide for a transistor gate structure. Such gate structures are only one of the growing number of semiconductor related material systems under development that require characterization at an unprecedented level of complexity. Such challenges are not limited to merely a desire for near-atomic and monolayer spatial resolution, but are magnified by the level of accuracy, precision and speed demanded by the semiconductor fabrication industry. There is a distinctive need to develop adequate characterization methods and systems. The ability to characterize materials at such levels is necessary to enable product development and also necessarily precedes evolution of process control. For example, there is a need for suitable systems and methods to provide parametric thickness, nitrogen dose, and nitrogen distribution information for thin nitrided silicon oxide films such as used for transistor gate oxides.